In recent years, zinc or zinc alloy-plating has been relatively often used as a method for preventing corrosion of metal surfaces. Plating alone, however, does not provide sufficient corrosion resistance, and thus a treatment with chromic acid, a so-called chromate treatment after the plating has been widely used. The chromate treatment here forms a chromate film using hexavalent chromium as a main component. However, it has recently been pointed out that hexavalent chromium may adversely affect the human body and the environment and there has been a strong trend to restrict the use of hexavalent chromium. Particularly, in the industrial field, alternative techniques to hexavalent chromate have been strongly needed.
Under such circumstances, a method for forming a trivalent chromium chemical conversion coating film has been proposed, which is free from hexavalent chromium (for example, U.S. Pat. No. 5,415,702, Japanese Patent Application Laid-Open (JP-A) No. 2003-166074, JP-A No. 2003-166075). Although having excellent corrosion resistance, a trivalent chromium chemical conversion coating film formed by this method has relatively less glossy appearance than conventional black hexavalent chromate films. In order to obtain higher gloss, the surface of the trivalent chromium chemical conversion coating film had to be overcoated with a thick resin film, silicate film, or the like again. Such overcoating, however, may cause problems with insulation and fastening performance. A treatment with a finishing agent for a trivalent chromium chemical conversion coating film, including a trivalent chromium source, a phosphate ion source, a zinc ion source, and a chelating agent capable of forming a complex with trivalent chromium (for example, JP-A No. 2005-23372) does not cause problems with insulation and fastening performance, but may cause a film formed by this treatment to have problems of many scratches due to the contact of articles, and green stains and white stains due to liquid accumulation or the like in a drying step of the finishing treatment, for example. There sometimes arises a problem of mold being generated in the finishing agent.